The influence of saturated and unsaturated fatty acids and fatty acyl coenzyme A thioesters on cholesterol synthesis in vitro has been studied in a rat liver post-mitochondrial supernatant system. 100 micronM free fatty acids do not influence in vitro cholesterol synthesis. Various fatty acyl-CoA thioesters at 10--100 microntm inhibit [14C]acetate incorporation into digitonin-precipitable sterols, the more unsaturated derivatives causing the greatest inhibition. 10 micronM arachidonoyl-CoA inhibits [14C]acetate incorporation into sterols 17% and 50 micronM inhibits 55%. [14C]Acetyl-CoA incorporation into sterols is similarly inhibited but [14C]mevalonate incorporation is not inhibited. Thus, the inhibition may be on the rate-controlling step of cholesterol synthesis, the conversion of beta-hydroxy-beta-methylglutaryl-CoA to mevalonate. Unsaturated fatty acyl-CoA thioesters may be important in regulating cholesterol synthesis. 2. Studies were undertaken to determine if the previously observed inhibition of cholesterol synthesis by thyroxine in vitro may relate to the thyroxine stimulation of fatty acid desaturation. 50 micronM thyroxine causes a preferential incorporation of [14C]acetate into unsaturated fatty acids while inhibiting acetate incorporation into sterols. However, a sufficient increase in unsaturated fatty acyl-CoA thioesters to account for the thyroxine inhibition of cholesterol synthesis was not demonstrated.
